High Strength Steel Exhibiting Good Ductility and Method of Production via Quenching and Partitioning Treatment by Zinc Bath

ABSTRACT

Steel with high strength and good formability is produced with compositions and methods for forming austenitic and martensitic microstructure in the steel. Carbon, manganese, molybdenum, nickel copper and chromium may promote the formation of room temperature stable (or meta-stable) austenite by mechanisms such as lowering transformation temperatures for non-martensitic constituents, and/or increasing the hardenability of steel. Thermal cycles utilizing a rapid cooling below a martensite start temperature followed by reheating may promote formation of room temperature stable austenite by permitting diffusion of carbon into austenite from martensite.

The present application claims priority from provisional patentapplication Ser. No. 61/824,643, entitled “High-Strength SteelExhibiting Good Ductility and Method of Production via In-LinePartitioning Treatment by Zinc Bath,” filed on May 17, 2013; andprovisional patent application Ser. No. 61/824,699, entitled“High-Strength Steel Exhibiting Good Ductility and Method of Productionvia In-Line Partitioning Treatment Downstream of Molten zinc Bath,”filed on May 17, 2013. The disclosures of application Ser. Nos.61/824,643, and 64/824,699 are incorporated herein by reference.

BACKGROUND

It is desirable to produce steels with high strength and goodformability characteristics. However, commercial production of steelsexhibiting such characteristics has been difficult due to factors suchas the desirability of relatively low alloying additions and limitationson thermal processing capabilities of industrial production lines. Thepresent invention relates to steel compositions and processing methodsfor production of steel using hot-dip galvanizing/galvannealing (HDG)processes such that the resulting steel exhibits high strength and coldformability.

SUMMARY

The present steel is produced using a composition and a modified HDGprocess that together produces a resulting microstructure consisting ofgenerally martensite and austenite (among other constituents). Toachieve such a microstructure, the composition includes certain alloyingadditions and the HDG process includes certain process modification, allof which are at least partially related to driving the transformation ofaustenite to martensite followed by a partial stabilization of austeniteat room-temperature.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate embodiments, and together withthe general description given above, and the detailed description of theembodiments given below, serve to explain the principles of the presentdisclosure.

FIG. 1 depicts a schematic view of a HDG temperature profile with apartitioning step performed after galvanizing/galvannealing.

FIG. 2 depicts a schematic view of a HDG temperature profile with apartitioning step performed during galvanizing/galvannealing.

FIG. 3 depicts a plot of one embodiment with Rockwell hardness plottedagainst cooling rate.

FIG. 4 depicts a plot of another embodiment with Rockwell hardnessplotted against cooling rate.

FIG. 5 depicts a plot of another embodiment with Rockwell hardnessplotted against cooling rate.

FIG. 6 depicts six photo micrographs of the embodiment of FIG. 3 takenfrom samples being cooled at various cooling rates.

FIG. 7 depicts six photo micrographs of the embodiment of FIG. 4 takenfrom samples being cooled at various cooling rates.

FIG. 8 depicts six photo micrographs of the embodiment of FIG. 5 takenfrom samples being cooled at various cooling rates.

FIG. 9 depicts a plot of tensile data as a function of austenitizationtemperature for several embodiments.

FIG. 10 depicts a plot of tensile data as a function of austenitizationtemperature for several embodiments.

FIG. 11 depicts a plot of tensile data as a function of quenchtemperature for several embodiments.

FIG. 12 depicts a plot of tensile data as a function of quenchtemperature for several embodiments.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of the thermal cycle used toachieve high strength and cold formability in a steel sheet having acertain chemical composition (described in greater detail below). Inparticular, FIG. 1 shows a typical hot-dip galvanizing or galvannealingthermal profile (10) with process modifications shown with dashed lines.In one embodiment the process generally involves austenitizationfollowed by a rapid cooling to a specified quench temperature topartially transform austenite to martensite, and the holding at anelevated temperature, a partitioning temperature, to allow carbon todiffuse out of martensite and into the remaining austenite, thus,stabilizing the austenite at room temperature. In some embodiments, thethermal profile shown in FIG. 1 may be used with conventional continuoushot-dip galvanizing or galvannealing production lines, although such aproduction line is not required.

As can be seen in FIG. 1, the steel sheet is first heated to a peakmetal temperature (12). The peak metal temperature (12) in theillustrated example is shown as being at least above the austenitetransformation temperature (A₁) (e.g., the dual phase, austenite+ferriteregion). Thus, at the peak metal temperature (12), at least a portion ofthe steel will be transformed to austenite. Although FIG. 1 shows thepeak metal temperature (12) as being solely above A₁, it should beunderstood that in some embodiments the peak metal temperature may alsoinclude temperatures above the temperature at which ferrite completelytransforms to austenite (A₃) (e.g., the single phase, austenite region).

Next the steel sheet undergoes rapid cooling. As the steel sheet iscooling, some embodiments may include a brief interruption in coolingfor galvanizing or galvannealing. In embodiments where galvanizing isused, the steel sheet may briefly maintain a constant temperature (14)due to the heat from the molten zinc galvanizing bath. Yet in otherembodiments, a galvannealing process may be used and the temperature ofthe steel sheet may be slightly raised to a galvannealing temperature(16) where the galvannealing process may be performed. Although, inother embodiments, the galvanizing or galvannealing process may beomitted entirely and the steel sheet may be continuously cooled.

The rapid cooling of the steel sheet is shown to continue below themartensite start temperature (M_(s)) for the steel sheet to apredetermined quench temperature (18). It should be understood that thecooling rate to M_(s) may be high enough to transform at least some ofthe austenite formed at the peak metal temperature (12) to martensite.In other words the cooling rate may be rapid enough to transformaustenite to martensite instead of other non-martensitic constituentssuch as ferrite, pearlite, or bainite which transform at relativelylower cooling rates.

As is shown in FIG. 1, the quench temperature (18) is below M_(s). Thedifference between the quench temperature (18) and M_(s) may varydepending on the individual composition of the steel sheet being used.However, in many embodiments the difference between quench temperature(18) and M_(s) may be sufficiently great to form an adequate amount ofmartensite to act as a carbon source to stabilize the austenite andavoid creating excessive “fresh” martensite upon final cooling.Additionally, quench temperature (18) may be sufficiently high to avoidconsuming too much austenite during the initial quench (e.g., to avoidexcessive carbon enrichment of austenite greater than that required tostabilize austenite for the given embodiment).

In many embodiments, quench temperature (18) may vary from about 191° C.to about 281° C., although no such limitation is required. Additionally,quench temperature (18) may be calculated for a given steel composition.For such a calculation, quench temperature (18) corresponds to theretained austenite having an M_(s) temperature of room temperature afterpartitioning. Methods for calculating quench temperature (18) are knownin the art and described in J. G. Speer, A. M. Streicher, D. K. Matlock,F. Rizzo, and G. Krauss, “Quenching And Partitioning : A FundamentallyNew Process to Create High Strength Trip Sheet Microstructures,”Austenite Formation and Decomposition, pp. 505-522, 2003; and A. M.Streicher, J. G. J. Speer, D. K. Matlock, and B. C. De Cooman,“Quenching and Partitioning Response of a Si-Added TRIP Sheet Steel,” inProceedings of the International Conference on Advanced High StrengthSheet Steels for Automotive Applications, 2004, the subject matter ofwhich is incorporated by reference herein.

The quench temperature (18) may be sufficiently low (with respect toM_(s)) to form an adequate amount of martensite to act as a carbonsource to stabilize the austenite and avoid creating excessive “fresh”martensite upon the final quench. Alternatively, the quench temperature(18) may be sufficiently high to avoid consuming too much austeniteduring the initial quench and creating a situation where the potentialcarbon enrichment of the retained austenite is greater than thatrequired for austenite stabilization at room temperature. In someembodiments, a suitable quench temperature (18) may correspond to theretained austenite having an M_(s) temperature of room temperature afterpartitioning. Speer and Streicher et al. (above) have providedcalculations that provide guidelines to explore processing options thatmay result in desirable microstructures. Such calculations assumeidealized full partitioning, and may be performed by applying theKoistinen-Marburger (KM) relationship twice (f_(m)=1−e^(−1.1×10) ⁻²^((ΔT)))—first to the initial quench to quench temperature (18) and thento the final quench at room temperature (as further described below).The Ms temperature in the KM expression can be estimated using empiricalformulae based on austenite chemistry (such as that of Andrew's linearexpression):

Ms(° C.)=539−423C−30.4Mn−7.5Si+30Al

The result of the calculations described by Speer et al. may indicate aquench temperature (18) which may lead to a maximum amount of retainedaustenite. For quench temperatures (18) above the temperature having amaximum amount of retained austenite, significant fractions of austeniteare present after the initial quench; however, there is not enoughmartensite to act as a carbon source to stabilize this austenite.Therefore, for the higher quench temperatures, increasing amounts offresh martensite form during the final quench. For quench temperaturesbelow the temperature having a maximum amount of retained austenite, anunsatisfactory amount of austenite may be consumed during the initialquench and there may be an excess amount of carbon that may partitionfrom the martensite.

Once the quench temperature (18) is reached, the temperature of thesteel sheet is either increased relative to the quench temperature ormaintained at the quench temperature for a given period of time. Inparticular, this stage may be referred to as the partitioning stage. Insuch a stage, the temperature of the steel sheet is at least maintainedat the quench temperature to permit carbon diffusion from martensiteformed during the rapid cooling and into any remaining austenite. Suchdiffusion may permit the remaining austenite to be stable (ormeta-stable) at room temperature, thus improving the mechanicalproperties of the steel sheet.

In some embodiments, the steel sheet may be heated above M_(s) to arelatively high partitioning temperature (20) and thereafter held at thehigh partitioning temperature (20). A variety of methods may be utilizedto heat the steel sheet during this stage. By way of example only, thesteel sheet may be heated using induction heating, torch heating, and/orthe like. Alternatively, in other embodiments, the steel sheet may beheated but to a different, lower partitioning temperature (22) which isslightly below M_(s). The steel sheet may then be likewise held at thelower partitioning temperate (22) for a certain period of time. In stilla third alternative embodiment, another alternative partitioningtemperature (24) may be used where the steel sheet is merely maintainedat the quench temperature. Of course, any other suitable partitioningtemperature may be used as will be apparent to those of ordinary skillin the art in view of the teachings herein.

After the steel sheet has reached the desired partitioning temperature(20, 22, 24), the steel sheet is maintained at the desired partitioningtemperature (20, 22, 24) for a sufficient time to permit partitioning ofcarbon from martensite to austenite. The steel sheet may then be cooledto room temperature.

FIG. 2 shows an alternative embodiment of the thermal cycle describedabove with respect to FIG. 1 (with a typical galvanizing/galvannealingthermal cycle shown with a solid line (40) and departures from typicalshown with a dashed line). In particular, like with the process of FIG.1, the steel sheet is first heated to a peak metal temperature (42). Thepeak metal temperature (42) in the illustrated embodiment is shown asbeing at least above A₁. Thus, at the peak metal temperature (42), atleast a portion of the steel sheet will be transformed to austenite. Ofcourse, like the process of FIG. 1, the present embodiment may alsoinclude a peak metal temperature in excess of A₃.

Next, the steel sheet may be rapidly quenched (44). It should beunderstood that the quench (44) may be rapid enough to initiatetransformation of some of the austenite formed at the peak metaltemperature (42) into martensite, thus avoiding excessive transformationto non-martensitic constituents such as ferrite, pearlite, banite,and/or the like.

The quench (44) may be then ceased at a quench temperature (46). Likethe process of FIG. 1, quench temperature (46) is below M_(s). Ofcourse, the amount below Ms may vary depending upon the material used.However, as described above, in many embodiments the difference betweenquench temperature (46) and M_(s) may be sufficiently great to form anadequate amount of martensite yet be sufficiently low to avoid consumingtoo much austenite.

The steel sheet is then subsequently reheated (48) to a partitioningtemperature (50, 52). Unlike the process of FIG. 1, the partitioningtemperature (50, 52) in the present embodiment may be characterized bythe galvanizing or galvannealing zinc bath temperature (if galvanizingor galvannealing is so used). For instance, in embodiments wheregalvanizing is used, the steel sheet may be re-heated to the galvanizingbath temperature (50) and subsequently held there for the duration ofthe galvanizing process. During the galvanizing process, partitioningmay occur similar to the partitioning described above. Thus, thegalvanizing bath temperature (50) may also function as the partitioningtemperature (50). Likewise, in embodiments where galvannealing is used,the process may be substantially the same with the exception of a higherbath/partitioning temperature (52).

Finally, the steel sheet is permitted to cool (54) to room temperaturewhere at least some austenite may be stable (or meta-stable) from thepartitioning step described above.

In some embodiments the steel sheet may include certain alloyingadditions to improve the propensity of the steel sheet to form aprimarily austenitic and martensitic microstructure and/or to improvethe mechanical properties of the steel sheet. Suitable compositions ofthe steel sheet may include one or more of the following, by weightpercent: 0.15-0.4% carbon, 1.5-4% manganese, 0-2% silicon or aluminum orsome combination thereof, 0-0.5% molybdenum, 0-0.05% niobium, otherincidental elements, and the balance being iron.

In addition, in other embodiments suitable compositions of the steelsheet may include one or more of the following, by weight percent:0.15-0.5% carbon, 1-3% manganese, 0-2% silicon or aluminum or somecombination thereof, 0-0.5% molybdenum, 0-0.05% niobium, otherincidental elements, and the balance being iron. Additionally, otherembodiments may include additions of vanadium and/or titanium inaddition to, or in lieu of niobium, although such additions are entirelyoptional.

In some embodiments carbon may be used to stabilize austenite. Forinstance, increasing carbon may lower the Ms temperature, lowertransformation temperatures for other non-martensitic constituents(e.g., bainite, ferrite, pearlite), and increase the time required fornon-martensitic products to form. Additionally, carbon additions mayimprove the hardenability of the material thus retaining formation ofnon-martensitic constituents near the core of the material where coolingrates may be locally depressed. However, it should be understood thatcarbon additions may be limited as significant carbon additions may leadto detrimental effects on weldability.

In some embodiments manganese may provide additional stabilization ofaustenite by lowering transformation temperatures of othernon-martensitic constituents, as described above. Manganese may furtherimprove the propensity of the steel sheet to form a primarily austeniticand martensitic microstructure by increasing hardenability.

In other embodiments molybdenum may be used to increase hardenability.

In other embodiments silicon and/or aluminum may be provided to reducethe formation of carbides. It should be understood that a reduction incarbide formation may be desirable in some embodiments because thepresence of carbides may decrease the levels of carbon available fordiffusion into austenite. Thus, silicon and/or aluminum additions may beused to further stabilize austenite at room temperature.

In some embodiments, nickel, copper, and chromium may be used tostabilize austenite. For instance, such elements may lead to a reductionin the M_(s) temperature. Additionally, nickel, copper, and chromium mayfurther increase the hardenability of the steel sheet.

In some embodiments niobium (or other micro-alloying elements, such astitanium, vanadium, and/or the like) may be used to increase themechanical properties of the steel sheet. For instance, niobium mayincrease the strength of the steel sheet through grain boundary pinningresulting from carbide formation.

In other embodiments, variations in the concentrations of elements andthe particular elements selected may be made. Of course, where suchvariations are made, it should be understood that such variations mayhave a desirable or undesirable effect on the steel sheet microstructureand/or mechanical properties in accordance with the properties describedabove for each given alloying addition.

EXAMPLE 1

Embodiments of the steel sheet were made with the compositions set forthin Table 1 below.

The materials were processed on laboratory equipment according to thefollowing parameters. Each sample was subjected to Gleeble 1500treatments using copper cooled wedge grips and the pocket jaw fixture.Samples were austenitized at 1100° C. and then cooled to roomtemperature at various cooling rates between 1-100° C./s.

TABLE 1 Chemical compositions in weight %. Descrip- ID tion Al C Co CrCu Mn Mo Nb Ni P Si Sn Ti V W V4037 Lab 1.41 0.19 — 0.01 <0.003 1.54<0.003 <0.003 <0.003 <0.003 0.11 <0.003 0.01 <0.003 — Material V4038 Lab1.29 0.22 — 0.20 <0.003 1.68 <0.003 0.02 <0.003 0.02 0.01 <0.003 0.01<0.003 — Material V4039 Lab <0.003 0.20 <0.002 0.01 <0.002 2.94 <0.0020.00 <0.002 0.00 1.57 <0.002 0.01 <0.002 0.00 Material

EXAMPLE 2

The Rockwell hardness of each of the steel compositions described inExample 1 and Table 1 above was taken on the surface of each sample. Theresults of the tests are plotted in FIGS. 3-5 with Rockwell hardnessplotted as a function of cooling rate. The average of at least sevenmeasurements is shown for each data point. The compositions V4037, V4038and V4039 correspond to FIGS. 3, 4, and 5, respectively.

EXAMPLE 3

Light optical micrographs were taken in the longitudinal throughthickness direction near the center of each sample for each of thecompositions of Example 1. The results of these tests are shown in FIGS.6-8. The compositions V4037, V4038, and V4039 correspond to FIGS. 6, 7,and 8, respectively. Additionally, FIGS. 6-8 each contain sixmicrographs for each composition with each micrograph representing asample subjected to a different cooling rate.

EXAMPLE 4

A critical cooling rate for each of the compositions of Example 1 wasestimated using the data of Examples 2 and 3 in accordance with theprocedure described herein. The critical cooling rate herein refers tothe cooling rate required to form martensite and avoid the formation ofnon-martensitic transformation products. The results of these tests areas follows:

V4037: 70° C./s

V4038: 75° C./s

V4039: 7° C./s

EXAMPLE 5

Embodiments of the steel sheet were made with the compositions set forthin Table 2 below.

The materials were processed by melting, hot rolling, and cold rolling.The materials were then subjected to testing described in greater detailbelow in Examples 6-7. All of the compositions listed in Table 2 wereintended for use with the process described above with respect to FIG. 2with the exception of V4039 which was intended for use with the processdescribed above with respect to FIG. 1. Heat V4039 had a compositionintended to provide higher hardenability as required by the thermalprofile described above with respect to FIG. 1. As a result V4039 wassubjected to annealing at 600° C. for 2 hours in 100% H2 atmosphereafter hot rolling, but prior to cold rolling. All materials were reducedduring cold rolling about 75% to 1 mm. Results for some of the materialcompositions set forth in Table 2 after hot rolling and cold rolling areshown in Tables 3 and 4, respectively.

TABLE 2 Chemical compositions in weight %. Heat Description C Mn Si AlMo Cr Nb B V4037 Lab Material 0.19 1.54 0.11 1.41 0 0.009 0 0.0007 V1307Lab Material 0.19 1.53 1.48 0.041 0 0 0 0.0005 V4063 Lab Material 0.191.6 0.11 1.34 0 0.003 0 0.0007 V4038 Lab Material 0.22 1.68 0.007 1.29 00.2 0.021 0.0008 V4039 Lab Material 0.2 2.94 1.57 <0.030 <0.002 0.0050.002 N/R V1305 Lab Material 0.2 2.94 1.57 0 0 0 0 0.0006 V4107 LabMaterial 0.18 4.03 1.63 0.005 0 0 0 0.0008 V4108 Lab Material 0.18 5.061.56 0.004 0 0 0 0.0009 V4060 Lab Material 0.4 1.2 1.97 0.003 0 0.190.007 0.0005 V4061 Lab Material 0.41 1.2 0.98 0.003 0 0.003 0 0.0004V4062 Lab Material 0.39 1.18 0.012 1.16 0 0.003 0 0.0007 V4078-1 LabMaterial 0.2 1.67 0.1 1.41 0.28 0.003 <0.003 0.0007 V4078-2 Lab Material0.2 1.67 0.1 1.41 0.27 <0.003 0.051 0.0007 V4078-1 Lab Material 0.191.94 0.098 1.43 <0.003 <0.003 <0.003 0.0007 V4078-2 Lab Material 0.191.96 0.099 1.41 <0.003 <0.003 0.051 0.0007

TABLE 3 Tensile Data, Post Hot Rolling Yield Strength Total Upper YSLower YS 0.2% Offset UTS Elongation Uniform Hardness Heat YPE (%) MPaksi MPa ksi MPa ksi MPa ksi (2″) Elongation % HRA V4063 0 N/A N/A N/AN/A 375 54 652 95 26 15 53 0 N/A N/A N/A N/A 380 55 648 94 26 15 53V4039* 0 N/A N/A N/A N/A 640 93 1085 157 14 9 67 V4039* 0 N/A N/A N/AN/A 603 88 748 109 20 10 61 (annealed) V4060 0.6 645 94 637 92 633 92883 128 20 11 63 0.5 610 89 605 88 611 89 876 127 22 12 61 V4061 0 N/AN/A N/A  0 496 72 790 115 22 11 60 0 N/A N/A N/A  0 507 74 799 116 20 1160 V4062 1.1 507 74 501 73 506 73 712 103 26 12 60 0.7 505 73 502 73 50273 713 103 24 12 57 V4078-1 0.8 427 62 416 60 425 62 594 86 32 18 51V4078-2 0.6 525 76 519 75 525 76 685 99 21 15 56 V4049-1 1.8 364 53 36152 361 52 544 79 30 17 48 V4079-2 1.2 497 72 481 70 489 71 639 93 24 1352 *Tensile test performed in transverse direction for V4039

TABLE 4 Tensile Data, Post Cold Rolling Yield Strength Total UniformHard- 0.2% Offset UTS Elonga- Elonga- ness Heat MPa ksi MPa ksi tion(2″) % tion % HRA V4037 927 134 971 141 4.8 1.4 64 V4063 1046 152 1101160 2.4 1.3 65 V4038 1001 145 1054 153 5.5 1.6 65 V4039 1149 167 1216176 4.4 1.5 68 V4060 1266 184 1393 202 5.4 1.9 69 V4061 1187 172 1279186 4.3 1.7 68 V4062 1111 161 1185 172 4.3 1.7 66 V4078-1 1047 152 1105160 3.6 1.4 65 V4078-2 1154 167 1209 175 4.2 1.4 66 V4079-1 932 135 975141 4.6 1.4 64 V4079-2 1034 150 1078 156 3.9 1.3 66

EXAMPLE 7

The compositions of Example 5 were subjected to Gleeble dilatomety.Gleeble dilatomety was performed in vacuum using a 101.6×25.4×1 mmsamples with a c-strain gauge measuring dilation in the 25.4 mmdirection. Plots were generated of the resulting dilation vs.temperature. Line segments were fit to the dilatometric data and thepoint at which the dilatometric data deviated from linear behavior wastaken as the transformation temperature of interest (e.g., A₁, A₃,M_(s)). The resulting transformation temperatures are tabulated in Table5.

Gleeble methods were also used to measure a critical cooling rate foreach of the compositions of Example 5. The first method utilized Gleebledilatomety, as described above. The second method utilized measurementsof Rockwell hardness. In particular, after samples were subjected toGleeble testing at range of cooling rates, Rockwell hardnessmeasurements were taken. Thus, Rockwell hardness measurements were takenfor each material composition with a measurement of hardness for a rangeof cooling rates. A comparison was then made between the Rockwellhardness measurements of a given composition at each cooling rate.Rockwell hardness deviations of 2 points HRA were consideredsignificant. The critical cooling rate to avoid non-martensitictransformation product was taken as the highest cooling rate for whichthe hardness was lower than 2 point HRA than the maximum hardness. Theresulting critical cooling rates are also tabulated in Table 5 for someof the compositions listed in Example 5.

TABLE 5 Transformation Temperatures and Critical Cooling Rate fromGleeble Dilatomety Critical Cooling Rate (° C./s) Gleeble Gleeble/ HeatA₁ (° C.) A₃ (° C.) M_(s) (° C.) Dilatometry Hardness V4037 737 970 469Inconclusive 65 V4063 720 975 425 70 — V4038 791 980 441 — 65 V4039 750874 394 <10   6 V4060 725 975 325 30 — V4061 675 900 325 40 55 V4062 700975 375 30 — V4078-1 750 925 450 40 55 V4078-2 790 980 425 40 — V4079-1800 1000 430 40 — V4079-2 750 990 425 40 —

EXAMPLE 8

The compositions of Example 5 were used to calculate quench temperatureand a theoretical maximum of retained austenite. The calculations wereperformed using the methods of Speer et al., described above. Theresults of the calculations are tabulated below in Table 6 for some ofthe compositions listed in Example 5.

TABLE 6 Quench Temperature and Theoretical Maximum of Retained Austenitef(γ) Theoretical Heat QT (° C.) Maximum V4037 281 0.15 V4063 278 0.15V4038 270 0.18 V4039 203 0.2 V4060 191 0.35 V4061 196 0.36 V4062 2370.31 V4078-1 276 0.16 V4078-2 276 0.16 V4079-1 273 0.16 V4079-2 272 0.16

EXAMPLE 9

The samples of the compositions of Example 5 were subjected to thethermal profiles shown in FIGS. 1 and 2 with peak metal temperature andquench temperature varied between samples of a given composition. Asdescribed above, only composition V4039 was subjected to the thermalprofile shown in FIG. 1, while all other compositions were subjected tothe thermal cycle shown in FIG. 2. For each sample, tensile strengthmeasurements were taken. The resulting tensile measurements are plottedin FIGS. 9-12. In particular, FIGS. 9-10 show tensile strength dataplotted against austenitization temperature and FIGS. 11-12 show tensilestrength data plotted against quench temperature. Additionally, wherethe thermal cycles were performed using Gleeble methods, such datapoints are denoted with “Gleeble.” Similarly, where thermal cycles wereperformed using a salt bath, such data points are denoted with “salt.”

Additionally, similar tensile measurements for each composition listedin Example 5 (where available) are tabulated in Table 7, shown below.Partitioning times and temperatures are shown for example only, in otherembodiments the mechanisms (such as carbon partitioning and/or phasetransformations) occur during non-isothermal heating and cooling to orfrom the stated partitioning temperature which may also contribute tofinal material properties.

TABLE 7 Tensile Data, Post Partitioning 0.2% Ultimate Total Peak MetalQuench Partitioning Partitioning Yield Tensile Elongation TE × UTS HeatTemp (° C.) Temp (° C.) Temp (° C.) Time (s) Strength Strength (%) (Mpa× %) V1307 800 250 466 30 419 818 27 22,424 800 250 466 30 416 807 2822,345 850 250 466 30 553 862 25 21,805 850 250 466 30 535 847 25 21,336900 250 466 30 548 854 24 20,144 800 250 400 30 445 898 22 19,675 900250 466 30 566 856 23 19,594 800 250 400 30 432 889 22 19,478 V4060 800160 466 15 746 1317 23 29,630 800 200 466 15 716 1332 19 25,309 800 250466 15 718 1403 18 25,115 800 200 466 15 632 1309 19 24,746 800 250 46615 701 1379 18 24,407 800 160 466 15 845 1311 18 23,986 850 250 466 15891 1291 18 23,749 850 250 466 15 735 1223 19 23,729 V4037 850 300 46615 443 657 32 20,763 921 200 466 30 325 612 34 20,633 850 250 466 15 405696 30 20,543 921 300 466 30 380 591 34 20,090 921 356 466 30 386 592 3420,078 921 400 466 30 388 588 34 19,937 940 200 466 30 362 598 33 19,906850 200 466 15 427 687 28 19,022 940 200 466 30 353 592 32 18,989 980200 466 30 341 612 31 18,897 900 300 466 15 493 727 26 18,767 850 200466 15 447 702 27 18,600 850 300 466 15 404 678 27 18,435 980 200 466 30347 611 30 18,387 940 200 466 30 330 548 33 18,253 980 200 466 30 345612 29 17,939 V4038 850 300 466 15 481 754 26 19,536 918 400 466 30 377681 27 18,461 918 286 466 30 357 695 26 18,348 918 200 466 30 363 697 2618,193 918 300 466 30 354 696 26 17,949 850 300 466 15 457 773 23 17,777V4039 800 250 400 60 821 1299 15 19,225 800 250 400 60 821 1298 1518,945 900 250 400 60 923 1273 15 18,593 850 250 400 60 874 1278 1418,142 900 250 400 60 913 1258 14 17,984 V4060 800 160 466 15 746 131723 29,630 800 200 466 15 716 1332 19 25,309 800 250 466 15 718 1403 1825,115 800 200 466 15 632 1309 19 24,746 800 250 466 15 701 1379 1824,407 800 160 466 15 845 1311 18 23,986 850 250 466 15 891 1291 1823,749 850 250 466 15 735 1223 19 23,729 800 200 466 30 942 1319 1722,422 850 200 466 15 695 1222 16 19,070 V4061 750 250 466 15 553 985 2019,902 750 250 466 15 581 918 21 18,996 V4062 750 200 466 15 478 813 2318,778 750 250 466 15 480 816 22 17,944 750 200 466 15 536 790 23 17,936V4107 850 250 400 60 776 1382 13 17,824 V4108 900 250 400 60 923 1642 1117,401 850 250 400 60 952 1620 11 17,337 V4078-1 850 300 466 15 448 78324 19,016 850 300 466 15 492 761 24 17,888 V4078-2 900 250 466 30 713843 21 17,946 850 300 466 15 689 859 20 17,525 850 300 466 15 671 871 2017,503

It will be understood various modifications may be made to thisinvention without departing from the spirit and scope of it. Therefore,the limits of this invention should be determined from the appendedclaims.

What is claimed is:
 1. A steel sheet comprising the following elementsby weight percent: 0.15-0.5% carbon; 1-3% manganese; 2% or less silicon,aluminum, or some combination thereof; 0.5% or less molybdenum; 0.05% orless niobium; and the balance being iron and other incidentalimpurities.
 2. A method for processing a steel sheet, the methodcomprising: (a) heating the steel sheet to a first temperature (T1),wherein T1 is at least above the temperature at which the steel sheettransforms to austenite and ferrite; (b) cooling the steel sheet to asecond temperature (T2) by cooling at a cooling rate, wherein T2 isbelow the martensite start temperature (M_(s)), wherein the cooling rateis sufficiently rapid to transform austenite to martensite; (c)re-heating the steel sheet to a partitioning temperature, wherein thepartitioning temperature is sufficient to permit diffusion of carbonwithin the structure of the steel sheet; (d) stabilizing austenite byholding the steel sheet at the partitioning temperature for a holdingtime, wherein the holding time is of a period of time sufficient topermit diffusion of carbon from martensite to austenite; and (e) coolingthe steel sheet to room temperature.
 3. The method of claim 2, furthercomprising hot dip galvanizing or galvannealing while stabilizingaustenite.
 4. The method of claim 3, wherein the hot dip galvanizing orgalvannealing occurs above M_(s).
 5. The method of claim 2, wherein thepartitioning temperature is above M_(s).
 7. The method of claim 2,wherein the steel sheet comprises the following elements by weightpercent: 0.15-0.4% carbon; 1.5-4% manganese; 2% or less silicon,aluminum, or some combination thereof; 0.5% or less molybdenum; 0.05% orless niobium; and the balance being iron and other incidentalimpurities.